A small amount of N2O (nitrous oxide) is contained in a waste gas of a wastewater sludge incinerator and a boiler of a thermal power plant, an exhaust gas of an automobile, or the like. N2O is one of greenhouse effect gases and has a global warming potential of 310 and causes the global warming effect 310 times as high as that of carbon dioxide. Therefore, in terms of global warming prevention, it is strongly desired to lessen the emission of N2O to the atmospheric air.
Accordingly, methods for reducing and removing N2O by using N2O decomposition catalysts have conventionally been proposed as a method for removing N2O from waste gases. For example, Patent Documents 1, 2, and 3 disclose methods for reducing and removing N2O using iron-zeolite based catalysts obtained by depositing iron or iron ion on zeolites. It is described that such iron-zeolite based catalysts show little deterioration of catalytic activity by NOx and SOx, in waste gases.
The above-mentioned iron-zeolite based catalysts are to be used in a temperature range of about 350 to 500° C. and a N2O decomposition catalyst to be used in a low temperature range is disclosed in Patent Document 4. This N2O decomposition catalyst is obtained by depositing a noble metal such as Rh, Ir, Pd, Pt, Ru, or the like on an alumina or zeolite carrier and is explained to be capable of decomposing N2O at 400° C. or lower.
All of these decomposition catalysts of Patent Documents 1 to 4 are for reducing and removing N2O by reaction of N2O with a reducing agent such as a hydrocarbon or ammonia in a temperature range of about 400° C. and the reaction formula is as follows.N2O+¼CH4→N2+¼CO2+½H2ON2O+⅔NH3→ 4/3N2+H2O
The above-mentioned Patent Documents 1 to 4 do not contain any description regarding means of controlling the addition. amount of a reducing agent or waste gas temperature. However, in actual plant facilities, the waste gas properties are always fluctuated significantly and for example, in a wastewater sludge incinerator, according to the change of the components of wastewater sludge to be loaded and water content, the steam concentration in a waste gas is considerably fluctuated. Further, the steam derived from hydrogen in a fuel is fluctuated along with the combustion condition.
When the steam amount in a waste gas is fluctuated as described, the activity of a N2O decomposition catalyst is fluctuated and thus the capability of N2O removal is considerably fluctuated. As a result, there is a problem that stable waste gas treatment cannot be carried out. Although the lowering of the catalytic activity of an iron-zeolite based catalyst due to steam is said to be relatively slight, considerable deterioration of the capability of N2O removal is inevitable also in the case of using the iron-zeolite based catalyst.    Patent Document 1: Japanese Patent No. 3550653    Patent Document 2: Japanese Patent No. 3681769    Patent Document 3: Japanese Patent Application Publication (Laid-Open) No. 2005-527350    Patent Document 4: Japanese Patent Application Laid-Open (JP-A) No. 2006-272240